I kept them separated with the ducks in a pen and a large dog crate to sleep in for a while because the chickens were basically full grown and the ducklings were still babies. Then we lost a couple chickens so we stopped free ranging them 100% and started keeping them in a pen more of the time while the ducks (3 hens, one drake) ranged. Then we lost our male duck, so there was no reason to keep them separate anymore, and we've had the chickens and ducks together ever since.
They sleep in separate coops overnight though. The chickens have food and water in their coop, the ducks only have bedding in their hut, so I have to move them into the pen in the morning for food and water. Everyone eats All Flock pellets now, with oyster shell on the side. I have young pullets and cockerels coming up too, so now I offer the starter crumble in the pen as well, which the hens do try to eat, but I don't mind. So long as everyone has the food they need, it works for me. The layers get their oyster shell on the side, the ducks get the niacin they need from the All Flock formula, and the babies can eat either starter/grower or All Flock. It's really all good, (except their starter / grower is probably higher quality than the All Flock, honestly,) but for me I'd so much rather just feed everyone the same thing and be done with it. lol